What this handout is about
This handout will help you write business letters required in many different situations, from applying for a job to requesting or delivering information. While the examples that are discussed specifically are the application letter and cover letter, this handout also highlights strategies for effective business writing in general.
Principles to keep in mind
Business writing is different
Writing for a business audience is usually quite different than writing in the humanities, social sciences, or other academic disciplines. Business writing strives to be crisp and succinct rather than evocative or creative; it stresses specificity and accuracy. This distinction does not make business writing superior or inferior to other styles. Rather, it reflects the unique purpose and considerations involved when writing in a business context.
When you write a business document, you must assume that your audience has limited time in which to read it and is likely to skim. Your readers have an interest in what you say insofar as it affects their working world. They want to know the "bottom line": the point you are making about a situation or problem and how they should respond.
Business writing varies from the conversational style often found in email messages to the more formal, legalistic style found in contracts. A style between these two extremes is appropriate for the majority of memos, emails, and letters. Writing that is too formal can alienate readers, and an attempt to be overly casual may come across as insincere or unprofessional. In business writing, as in all writing, you must know your audience.
In most cases, the business letter will be the first impression that you make on someone. Though business writing has become less formal over time, you should still take great care that your letter's content is clear and that you have proofread it carefully.
Pronouns and active versus passive voice
Personal pronouns (like I, we, and you) are important in letters and memos. In such documents, it is perfectly appropriate to refer to yourself as I and to the reader as you. Be careful, however, when you use the pronoun we in a business letter that is written on company stationery, since it commits your company to what you have written. When stating your opinion, use I; when presenting company policy, use we.
The best writers strive to achieve a style that is so clear that their messages cannot be misunderstood. One way to achieve a clear style is to minimize your use of the passive voice. Although the passive voice is sometimes necessary, often it not only makes your writing dull but also can be ambiguous or overly impersonal. Here's an example of the same point stated in passive voice and in the active voice:
PASSIVE: The net benefits of subsidiary divestiture were grossly overestimated.
[Who did the overestimating?]
ACTIVE: The Global Finance Team grossly overestimated the net benefits of subsidiary divestiture.
The second version is clearer and thus preferable.
Of course, there are exceptions to every rule. What if you are the head of the Global Finance Team? You may want to get your message across without calling excessive attention to the fact that the error was your team's fault. The passive voice allows you to gloss over an unflattering point—but you should use it sparingly.
Focus and specificity
Business writing should be clear and concise. Take care, however, that your document does not turn out as an endless series of short, choppy sentences. Keep in mind also that "concise" does not have to mean "blunt"—you still need to think about your tone and the audience for whom you are writing. Consider the following examples:
After carefully reviewing this proposal, we have decided to prioritize other projects this quarter.
Nobody liked your project idea, so we are not going to give you any funding.
The first version is a weaker statement, emphasizing facts not directly relevant to its point. The second version provides the information in a simple and direct manner. But you don't need to be an expert on style to know that the first phrasing is diplomatic and respectful (even though it's less concise) as compared with the second version, which is unnecessarily harsh and likely to provoke a negative reaction.
Business letters: where to begin
Reread the description of your task (for example, the advertisement of a job opening, instructions for a proposal submission, or assignment prompt for a course). Think about your purpose and what requirements are mentioned or implied in the description of the task. List these requirements. This list can serve as an outline to govern your writing and help you stay focused, so try to make it thorough. Next, identify qualifications, attributes, objectives, or answers that match the requirements you have just listed. Strive to be exact and specific, avoiding vagueness, ambiguity, and platitudes. If there are industry- or field-specific concepts or terminology that are relevant to the task at hand, use them in a manner that will convey your competence and experience. Avoid any language that your audience may not understand. Your finished piece of writing should indicate how you meet the requirements you've listed and answer any questions raised in the description or prompt.
Application letters and cover letters
Many people believe that application letters and cover letters are essentially the same. For purposes of this handout, though, these kinds of letters are different. The letter of application is a sales letter in which you market your skills, abilities, and knowledge. A cover letter, on the other hand, is primarily a document of transmittal. It identifies an item being sent, the person to whom it is being sent, and the reason for its being sent, and provides a permanent record of the transmittal for both the writer and the reader.
Application letters
When writing an application letter, remember that you probably have competition. Your audience is a professional who screens and hires job applicants—someone who may look through dozens or even hundreds of other applications on the day she receives yours. The immediate objective of your application letter and accompanying resume is to attract this person's attention. Your ultimate goal is to obtain an interview.
As you write your application letter, be sure you complete three tasks: catch the reader's attention favorably, convince the reader that you are a qualified candidate for the job, and request an interview.
Application letter checklist:
* Identify the job by title and let the recipient know how you heard about it.
* Summarize your qualifications for the job, specifically your work experience, activities that show your leadership skills, and your educational background.
* Refer the reader to your enclosed resume.
* Ask for an interview, stating where you can be reached and when you will be available. If your prospective employer is located in another city and you plan to visit the area, mention the dates for your trip.
* If you are applying for a specific job, include any information pertinent to the position that is not included in your resume.
To save your reader time and to call attention to your strengths as a candidate, state your objective directly at the beginning of the letter.
Example: I am seeking a position as a manager in your Data Center. In such a management position, I can use my master's degree in information systems and my experience as a programmer/analyst to address business challenges in data processing.
If you have been referred to a company by one of its employees, a career counselor, a professor, or someone else, mention that before stating your job objective.
Example: During the recent ARRGH convention in Washington, D.C., one of your sales representatives, Dusty Brown, informed me of a possible opening for a manager in your Data Center. My extensive background in programming and my master's degree in information systems make me highly qualified for the position.
In subsequent paragraphs, expand on the qualifications you mentioned in your opening. Add any appropriate details, highlighting experience listed on your resume that is especially pertinent to the job you are seeking. Close with a request for an interview. Proofread your letter carefully.
Two sample letters of application are presented below. The first letter (Sample #1) is by a recent college graduate responding to a local newspaper article about the company's plan to build a new computer center. The writer is not applying for a specific job opening but describes the position he seeks. The second letter (Sample #2) is from a college senior who does not specify where she learned of the opening because she is uncertain whether a position is available.
Sample #1
6123 Farrington Road
Apt. B11
Chapel Hill, NC 27514
January 11, 2005
Taylor, Inc.
694 Rockstar Lane
Durham, NC 27708
Dear Human Resources Director:
I just read an article in the News and Observer about Taylor's new computer center just north of Durham. I would like to apply for a position as an entry-level programmer at the center.
I understand that Taylor produces both in-house and customer documentation. My technical writing skills, as described in the enclosed resume, are well suited to your company. I am a recent graduate of DeVry Institute of Technology in Atlanta with an Associate's Degree in Computer Science. In addition to having taken a broad range of courses, I served as a computer consultant at the college's computer center where I helped train users to work with new systems.
I will be happy to meet with you at your convenience and discuss how my education and experience match your needs. You can reach me at my home address, at (919) 233-1552, or at krock@devry.alumni.edu.
Sincerely,
Raymond Krock
Sample #2
6123 Farrington Road
Apt. G11
Chapel Hill, NC 27514
January 11, 2005
Taylor, Inc.
694 Rockstar Lane
Durham, NC 27708
Dear Ms. Jones:
I am seeking a position in your engineering department where I may use my training in computer sciences to solve Taylor's engineering problems. I would like to be a part of the department that developed the Internet Selection System but am unsure whether you have a current opening.
I expect to receive a Bachelor of Science degree in Engineering from North Carolina State University in June and by that time will have completed the Computer Systems Engineering Program. Since September 2000, I have been participating, through the University, in the Professional Training Program at Computer Systems International in Raleigh. In the program I was assigned to several staff sections as an apprentice. Most recently, I have been a programmer trainee in the Engineering Department and have gained a great deal of experience in computer applications. Details of the academic courses I have taken are included in the enclosed resume.
If there is a position open at Taylor Inc., please let me know whom I should contact for further information. I look forward to hearing from you soon. I may be reached at my office (919-866-4000 ext. 232) or via email (Brock@aol.com).
Sincerely,
Rebecca Brock
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Cover letters
As mentioned previously, application letters and cover letters are not the same. A cover letter identifies an item being sent, the person to whom it is being sent, and the reason for its being sent. A cover letter provides a permanent record of the transmittal for both the writer and the reader.
In a cover letter, keep your remarks brief. Your opening should explain what you are sending and why. In an optional second paragraph, you might include a summary of the information you are sending. A letter accompanying a proposal, for example, might point out sections in the proposal that might be of particular interest to the reader. The letter could then go on to present a key point or two explaining why the writer's firm is the best one for the job. The closing paragraph should contain acknowledgements, offer additional assistance, or express the hope that the material will fulfill its purpose.
The following are examples of cover letters. The first letter (Sample #1) is brief and to the point. The second letter (Sample #2) is slightly more detailed because it touches on the manner in which the information was gathered.
Sample #1
Your Company Logo and Contact Information
January 11, 2005
Brian Eno, Chief Engineer
Carolina Chemical Products
3434 Pond View Lane
Durham, NC 27708
Dear Mr. Eno:
Enclosed is the final report on our installment of pollution control equipment at Eastern Chemical Company, which we send with Eastern's Permission. Please call me collect (ext. 1206) or email me at the address below if I can answer any questions.
Sincerely,
Nora Cassidy
Technical Services Manager
ncassidy@company.com
Enclosure: Report
Sample #2
Your Company Logo and Contact Information
January 11, 2005
Brian Eno, Chief Engineer
Ecology Systems, Inc.
8458 Obstructed View Lane
Durham, NC 27708
Dear Mr. Eno:
Enclosed is the report estimating our power consumption for the year as requested by John Brenan, Vice President, on September 4.
The report is the result of several meetings with Jamie Anson, Manager of Plant Operations, and her staff and an extensive survey of all our employees. The survey was delayed by the transfer of key staff in Building A. We believe, however, that the report will provide the information you need to furnish us with a cost estimate for the installation of your Mark II Energy Saving System.
We would like to thank Billy Budd of ESI for his assistance in preparing the survey. If you need more information, please let me know.
Sincerely,
Nora Cassidy
New Projects Office
ncassidy@company.com
Enclosure: Report
Monday, June 28, 2010
Letterhead. This should carry the company’s address and the name of the person who writes the letter. In the absence of such a company’s letterhead, A4 white paper will suffice. In that case, the complete postal address of the company and the name of the person who writes the letter should be clearly mentioned. The absence of this information will create suspicion in the addressee. This kind of negative image should be avoided at any cost in order to maintain the dignity of the company.
Date. Include the date so that the letter will be kept for future reference. This date-matter will give the addressee a fair idea about the urgency, if any, of the matter that the letter carries.
Recipients Name and Address. Add the address of the person you're sending the letter to. His/her designation should be mentioned so that the letter will get the touch of professionalism. If a letter is addressed to an individual rather than to the designated person, in the future the letter may go invalid.
Reference Number. Include a reference number, if necessary (if the letter is in response to any other previous letter, the date of the previous letter can be treated as the reference number). In the absence of the reference number and in the case of drafting a new letter, you should mention in the first paragraph itself the subject matter of the letter. You need not present all the matters in depth in the first paragraph itself. Just a brief introduction will do.
Salutation. Use a proper salutation. Addressing the recipient by name is preferred. Use the person's title (Mr. Mrs. Ms. or Dr.) with either a first name or a last name, but not both. Using a last name is more formal and should be used unless you are on first-name terms with the recipient.
Body. Introduce the topic at hand in the first paragraph.
# In the next paragraph, write the subject matter clearly in a simple language and in an unambiguous style. Since a business letter is only a tool to make few business-things done, brevity is what all the business-people expect in the letter that they receive.
# Be brief and direct in your communication. Whether you purchase goods or you introduce your company or you bring the mistake in the accounts to the attention of the addressee, you should be brief.
# Use the final paragraph to wind up the letter. Never hesitate to be forceful in your communication. Any lethargic approach will not be tolerated by the addressee. Only forceful and firm people are respected by the eminent business people.
Closing. Use the correct form of leave-taking. Yours sincerely and yours cordially are the widely used forms of leave taking. Use any one of them.
Signature
Signature.Add your signature in the proper place. The letter without the signature of the writer will be treated by the addressee. It carries no weight. Please keep in mind to add your official signature before you put the letter in the postal cover.
Authors Name and Contact Info. Enter the name of the letters author and any pertinent contact information.
# Check the spelling of your letter. You can use your computer to do this job. But do not rely on the computer software -- read it over yourself.
Use a clean and appropriate envelope, preferably with your business' name on it. Write the addresses of your addressee and yours in the appropriate places.
Paste adequate stamps. If not, the letter may bounce.
teps
* Make your letter a computer typed one so that if your hand writing is not readable, that will not affect the effectiveness of the letter.
* Use a quality pen to sign the letter.
* Post the letter in time.
* Be Responsive - If you are responding to or with a letter, address the inquiry or problem. Most of the time, companies rely too much on a handful of form letters to answer all situations. This shows that you do not understand their needs. When you considered you reader as above, you will be able to respond to them.
* Try using the "7 Cs":
o Be Clear: Let your reader know exactly what you are trying to say. Your reader will only respond quickly if your meaning is crystal clear. In particular, if there is some result or action you want taken because of your letter, state what it is.
o Be Conversational: Letters are written by people to people. Don't address it "to whom it may concern" if it is possible. Whatever you do, do not use a photocopied form letter. Please see how to use a form letter for the proper use of form letter if you have to use it. You cannot build a relationship with canned impersonal letters. But also don't be too informal. Avoid using colloquial language or slang such as "you know" or "I mean" or "wanna". Keep the tone businesslike, but be friendly and helpful.
o Be Courteous: Let your reader know exactly what you are trying to say. Your reader will only respond quickly if your meaning is crystal clear. Even if you are writing with a complaint or concern, you can be courteous.
o Be Concise and to the point: When writing business letter, explain your position in as few words as possible. Spell out what you can do and what they need to do. Use clear and easy to understand language so that any misunderstanding can be minimized. Think before you write. Ask yourself why you are writing? What is it that you want to achieve?
o Be Correct. Take the time to make sure you have the facts straight before putting them in writing. Check your spelling and grammar, too, or have someone check them for you.
o Be Convincing. Most likely the purpose of your letter is to persuade your reader to do something: change their mind, correct a problem, send money, or take action. Make your case.
o Be Complete. Don't omit necessary information.
* Use this 5 step process to organize your letter and keep it brief:
1. List out the topics you want to cover. Do not worry about the order.
2. In each topic, list keywords, examples, arguments and facts.
3. Review each topic in your outline for relevance to your aim and audience.
4. Cut out anything that's not relevant.
5. Sort the information into the best order for your readers.
* Be friendly, build the relationship - Don't use cold, formal language. Some people have the perception that when writing business letter, they must big words. To them this is a sign of literacy. Some 'big words' have no substitute, but do use the word correctly. You want the reader to feel like they are reading a letter from someone who cares.
* Emphasize the positive - Talk about what you can do, not what you can't. For example, if a product is out of stock, don't tell the customer you are unable to fill the order, instead, tell them the product is very popular and you have sold out. Then tell them when you can get the order to them.
o Stay away from negative words. For example, your complaint about our product, instead, sorry our product was not up to your expectations.
* Be Prompt - If you cannot respond fully in less than a week, tell them so and say when they can expect a response from you.
Warnings
* Do not use abusive language.
* Do not use the word I too often in the letter.
Date. Include the date so that the letter will be kept for future reference. This date-matter will give the addressee a fair idea about the urgency, if any, of the matter that the letter carries.
Recipients Name and Address. Add the address of the person you're sending the letter to. His/her designation should be mentioned so that the letter will get the touch of professionalism. If a letter is addressed to an individual rather than to the designated person, in the future the letter may go invalid.
Reference Number. Include a reference number, if necessary (if the letter is in response to any other previous letter, the date of the previous letter can be treated as the reference number). In the absence of the reference number and in the case of drafting a new letter, you should mention in the first paragraph itself the subject matter of the letter. You need not present all the matters in depth in the first paragraph itself. Just a brief introduction will do.
Salutation. Use a proper salutation. Addressing the recipient by name is preferred. Use the person's title (Mr. Mrs. Ms. or Dr.) with either a first name or a last name, but not both. Using a last name is more formal and should be used unless you are on first-name terms with the recipient.
Body. Introduce the topic at hand in the first paragraph.
# In the next paragraph, write the subject matter clearly in a simple language and in an unambiguous style. Since a business letter is only a tool to make few business-things done, brevity is what all the business-people expect in the letter that they receive.
# Be brief and direct in your communication. Whether you purchase goods or you introduce your company or you bring the mistake in the accounts to the attention of the addressee, you should be brief.
# Use the final paragraph to wind up the letter. Never hesitate to be forceful in your communication. Any lethargic approach will not be tolerated by the addressee. Only forceful and firm people are respected by the eminent business people.
Closing. Use the correct form of leave-taking. Yours sincerely and yours cordially are the widely used forms of leave taking. Use any one of them.
Signature
Signature.Add your signature in the proper place. The letter without the signature of the writer will be treated by the addressee. It carries no weight. Please keep in mind to add your official signature before you put the letter in the postal cover.
Authors Name and Contact Info. Enter the name of the letters author and any pertinent contact information.
# Check the spelling of your letter. You can use your computer to do this job. But do not rely on the computer software -- read it over yourself.
Use a clean and appropriate envelope, preferably with your business' name on it. Write the addresses of your addressee and yours in the appropriate places.
Paste adequate stamps. If not, the letter may bounce.
teps
* Make your letter a computer typed one so that if your hand writing is not readable, that will not affect the effectiveness of the letter.
* Use a quality pen to sign the letter.
* Post the letter in time.
* Be Responsive - If you are responding to or with a letter, address the inquiry or problem. Most of the time, companies rely too much on a handful of form letters to answer all situations. This shows that you do not understand their needs. When you considered you reader as above, you will be able to respond to them.
* Try using the "7 Cs":
o Be Clear: Let your reader know exactly what you are trying to say. Your reader will only respond quickly if your meaning is crystal clear. In particular, if there is some result or action you want taken because of your letter, state what it is.
o Be Conversational: Letters are written by people to people. Don't address it "to whom it may concern" if it is possible. Whatever you do, do not use a photocopied form letter. Please see how to use a form letter for the proper use of form letter if you have to use it. You cannot build a relationship with canned impersonal letters. But also don't be too informal. Avoid using colloquial language or slang such as "you know" or "I mean" or "wanna". Keep the tone businesslike, but be friendly and helpful.
o Be Courteous: Let your reader know exactly what you are trying to say. Your reader will only respond quickly if your meaning is crystal clear. Even if you are writing with a complaint or concern, you can be courteous.
o Be Concise and to the point: When writing business letter, explain your position in as few words as possible. Spell out what you can do and what they need to do. Use clear and easy to understand language so that any misunderstanding can be minimized. Think before you write. Ask yourself why you are writing? What is it that you want to achieve?
o Be Correct. Take the time to make sure you have the facts straight before putting them in writing. Check your spelling and grammar, too, or have someone check them for you.
o Be Convincing. Most likely the purpose of your letter is to persuade your reader to do something: change their mind, correct a problem, send money, or take action. Make your case.
o Be Complete. Don't omit necessary information.
* Use this 5 step process to organize your letter and keep it brief:
1. List out the topics you want to cover. Do not worry about the order.
2. In each topic, list keywords, examples, arguments and facts.
3. Review each topic in your outline for relevance to your aim and audience.
4. Cut out anything that's not relevant.
5. Sort the information into the best order for your readers.
* Be friendly, build the relationship - Don't use cold, formal language. Some people have the perception that when writing business letter, they must big words. To them this is a sign of literacy. Some 'big words' have no substitute, but do use the word correctly. You want the reader to feel like they are reading a letter from someone who cares.
* Emphasize the positive - Talk about what you can do, not what you can't. For example, if a product is out of stock, don't tell the customer you are unable to fill the order, instead, tell them the product is very popular and you have sold out. Then tell them when you can get the order to them.
o Stay away from negative words. For example, your complaint about our product, instead, sorry our product was not up to your expectations.
* Be Prompt - If you cannot respond fully in less than a week, tell them so and say when they can expect a response from you.
Warnings
* Do not use abusive language.
* Do not use the word I too often in the letter.
Phases of The Moon
The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:
+ New Moon > New Crescent > First Quarter > Waxing Gibbous > Full Moon >
Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)
The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon's 29.5 day period?
To many early civilizations, the Moon's monthly cycle was an important tool for measuring the passage of time. In fact many calendars are synchronized to the phases of the Moon. The Hebrew, Muslem and Chinese calendars are all lunar calendars. The New Moon phase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moon's monthly cycle. When the Moon is New, it rises and sets with the Sun because it lies very close to the Sun in the sky. Although we cannot see the Moon during New Moon phase, it has a very special significance with regard to eclipses.
The Moon's Two Shadows
An eclipse of the Sun (or solar eclipse) can only occur at New Moon when the Moon passes between Earth and Sun. If the Moon's shadow happens to fall upon Earth's surface at that time, we see some portion of the Sun's disk covered or 'eclipsed' by the Moon. Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn't happen because the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region.
The Moon's shadow actually has two parts:
1. Penumbra
+ The Moon's faint outer shadow.
+ Partial solar eclipses are visible from within the penumbral shadow.
2. Umbra
+ The Moon's dark inner shadow.
+ Total solar eclipses are visible from within the umbral shadow.
When the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region. Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright. You must use special filters or a home-made pinhole projector to safely watch a partial eclipse of the Sun (see: Observing Solar Eclipses Safely).
What is the difference between a solar eclipse and a lunar eclipse? A lunar eclipse is an eclipse of the Moon rather than the Sun. It happens when the Moon passes through Earth's shadow. This is only possible when the Moon is in the Full Moon phase. For more information, see Lunar Eclipses for Beginners.
Total Solar Eclipses and the Path of Totality
If the Moon's inner or umbral shadow sweeps across Earth's surface, then a total eclipse of the Sun is seen. The track of the Moon's umbral shadow across Earth is called the Path of Totality. It is typically 10,000 miles long but only about 100 miles wide. It covers less than 1% of Earth's entire surface area. In order to see the Sun become completely eclipsed by the Moon, you must be somewhere inside the narrow path of totality.
The path of a total eclipse can cross any part of Earth. Even the North and South Poles get a total eclipse sooner or later. Just one total eclipse occurs each year or two. Since each total eclipse is only visible from a very narrow track, it is rare to see one from any single location. You'd have to wait an average of 375 years to see two total eclipses from one place. Of course, the interval between seeing two eclipses from one particular place can be shorter or longer. For instance, the last total eclipse visible from Princeton, NJ was in 1478 and the next is in 2079. That's an interval of 601 years. However, the following total eclipse from Princeton is in 2144, after a period of only 65 years.
Awesome Totality
The total phase of a solar eclipse is very brief. It rarely lasts more than several minutes. Nevertheless, it is considered to be one of the most awe inspiring spectacles in all of nature. The sky takes on an eerie twilight as the Sun's bright face is replaced by the black disk of the Moon. Surrounding the Moon is a beautiful gossemer halo. This is the Sun's spectacular solar corona, a super heated plasma two million degrees in temperature. The corona can only be seen during the few brief minutes of totality. To witness such an event is a singularly memorable experience which cannot be conveyed adequately through words or photographs. Nevertheless, you can read more about the Experience of Totality in the first chapter of Totality - Eclipses of the Sun.
Annular Solar Eclipses
Unfortunately, not every eclipse of the Sun is a total eclipse. Sometimes, the Moon is too small to cover the entire Sun's disk. To understand why, we need to talk about the Moon's orbit around Earth. That orbit is not perfectly round but is oval or elliptical in shape. As the Moon orbits our planet, it's distance varies from about 221,000 to 252,000 miles. This 13% variation in the Moon's distance makes the Moon's apparent size in our sky vary by the same amount. When the Moon is on the near side of its orbit, the Moon appears larger than the Sun. If an eclipse occurs at that time, it will be a total eclipse. However, if an eclipse occurs while the Moon is on the far side of its orbit, the Moon appears smaller than the Sun and can't completely cover it. Looking down from space, we would see that the Moon's umbral shadow is not long enough to reach Earth. Instead, the antumbra shadow reaches Earth.
The track of the antumbra is called the path of annularity. If you are within this path, you will see an eclipse where a ring or annulus of bright sunlight surrounds the Moon at the maximum phase. Annular eclipses are also dangerous to look directly with the naked eye. You must use the same precautions needed for safely viewing a partial eclipse of the Sun (see: Observing Solar Eclipses Safely).
Annularity can last as long as a dozen minutes, but is more typically about half that length. Since the annular phase is so bright, the Sun's gorgeous corona remains hidden from view. But annular eclipses are still quite interesting to watch. You can read reports about the annular eclipses of 1999 in Australia and 2003 in Iceland. More recently, visit the 2005 Annular Solar Eclipse Photo Gallery.
The "Oddball" Hybrid Eclipse
There's one more type of solar eclipse to mention and its a real oddball. Under rare circumstances, a total eclipse can change to an annular eclipse or vice versa along different sections of the eclipse path. This happens when the curvature of Earth brings different points of the path into the umbral (total) and antumbral (annular) shadows, respectively. Hybrid eclipses are sometimes called annular/total eclipses. The last hybrid eclipse was in 2005 and the next one is in 2013.
Solar Eclipse Frequency and Future Eclipses
During the five thousand year period 2000 BCE to 3000 CE, planet Earth experiences 11,898 solar eclipses as follows:
Scientists welcome the total eclipse as a rare opportunity to study the Sun's faint corona. Why is the corona so hot? What causes it to spew massive bubbles of plasma into space through coronal mass ejections? Can solar flares be predicted and what causes them? These major mysteries may eventually be solved through experiments performed at future total eclipses.
For amateur astronomers and eclipse chasers, an eclipse of the Sun presents a tempting target to photograph. Fortunately, Solar Eclipse Photography is easy provided that you have the right equipment and use it correctly. See MrEclipse's Picks for camera, lens and tripod recommendations. For more photographs taken during previous lunar eclipses, be sure to visit Solar Eclipse Photo Gallery. It's also possible to capture a solar eclipse using a video camcorder.
The most recent total solar eclipse occurred on March 29, 2006 and was visible from Africa and central Asia. Fred Espenak led a Spears Travel tour to Libya to witness the event. You can see a collection of his photographs at 2006 Eclipse Gallary. Reports (with photos) from some of his earlier eclipse expeditions include 2001 Eclipse in Zambia, 1999 Eclipse in Turkey, 1998 Eclipse in Aruba and 1995 Eclipse in India.
The next two total eclipse of the Sun are both visible from China: 2008 and 2009. Join Fred Espenak on a Spears Travel tour to witness one (or both!) of these spectacular events.
Eclipse conditions
Eclipses may occur when the Earth and the Moon are aligned with the Sun, and the shadow of one body cast by the Sun falls on the other. So at new moon (or rather Dark Moon), when the Moon is in conjunction with the Sun, the Moon may pass in front of the Sun as seen from a narrow region on the surface of the Earth and cause a solar eclipse. At full moon, when the Moon is in opposition to the Sun, the Moon may pass through the shadow of the Earth, and a lunar eclipse is visible from the night half of the Earth.
Note: Conjunction and opposition of the Moon together have a special name: syzygy (from Greek for "junction"), because of the importance of these lunar phases.
An eclipse does not happen at every new or full moon, because the plane of the orbit of the Moon around the Earth is tilted with respect to the plane of the orbit of the Earth around the Sun (the ecliptic): so as seen from the Earth, when the Moon is nearest to the Sun (new moon) or at largest distance (full moon), the three bodies usually are not exactly on the same line.
This inclination is on average about:
A lunar eclipse occurs when the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse. The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes. The next total lunar eclipse will occur on December 21, 2010. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place. Some lunar eclipses have been associated with important historical events.
Types of lunar eclipses
A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.
A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth’s umbra, one observes a total lunar eclipse. The Moon’s speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the Moon’s first and last contact with the shadow is much longer, and could last up to 3.8 hours.[1] The relative distance of the Moon from the Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease much with distance. Thus, a totally eclipsed Moon occurring near apogee will lengthen the duration of totality.
A selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time. This can only happen just before sunset or just after sunrise, and both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being referred to as a horizontal eclipse. It happens during every lunar eclipse at all those places on the Earth where it is sunrise or sunset at the time. Indeed, the reddened light that reaches the Moon comes from all the simultaneous sunrises and sunsets on the Earth. Although the Moon is in the Earth’s geometrical shadow, the Sun and the eclipsed Moon can appear in the sky at the same time because the refraction of light through the Earth’s atmosphere causes objects near the horizon to appear higher in the sky than their true geometric position.
The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth’s atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color; an alternative way of considering the problem is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth.
The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.
Eclipse cycles
Every year there are at least two lunar eclipses, although total lunar eclipses are significantly less common. If one knows the date and time of an eclipse, it is possible to predict the occurrence of other eclipses using an eclipse cycle like the Saros cycle.
Eclipses of the Sun: 2009 - 2015
Calendar Date Eclipse Type Eclipse Magnitude Central Duration Geographic Region of Eclipse Visibility
(Link to Global Map) (Link to Google Map) (Link to Path Table)
2009 Jan 26 Annular 0.928 07m54s s Africa, Antarctica, se Asia, Australia
[Annular: s Indian, Sumatra, Borneo]
2009 Jul 22 Total 1.080 06m39s e Asia, Pacific Ocean, Hawaii
[Total: India, Nepal, China, c Pacific]
2010 Jan 15 Annular 0.919 11m08s Africa, Asia
[Annular: c Africa, India, Malymar, China]
2010 Jul 11 Total 1.058 05m20s s S. America
[Total: s Pacific, Easter Is., Chile, Argentina]
2011 Jan 04 Partial 0.858 - Europe, Africa, c Asia
2011 Jun 01 Partial 0.601 - e Asia, n N. America, Iceland
2011 Jul 01 Partial 0.097 - s Indian Ocean
2011 Nov 25 Partial 0.905 - s Africa, Antarctica, Tasmania, N.Z.
2012 May 20 Annular 0.944 05m46s Asia, Pacific, N. America
[Annular: China, Japan, Pacific, w U.S.]
2012 Nov 13 Total 1.050 04m02s Australia, N.Z., s Pacific, s S. America
[Total: n Australia, s Pacific]
2013 May 10 Annular 0.954 06m03s Australia, N.Z., c Pacific
[Annular: n Australia, Solomon Is., c Pacific]
2013 Nov 03 Hybrid 1.016 01m40s e Americas, s Europe, Africa
[Hybid: Atlantic, c Africa]
2014 Apr 29 Annular 0.987 - s Indian, Australia, Antarctica
[Annular: Antarctica]
2014 Oct 23 Partial 0.811 - n Pacific, N. America
2015 Mar 20 Total 1.045 02m47s Iceland, Europe, n Africa, n Asia
[Total: n Atlantic, Faeroe Is, Svalbard]
2015 Sep 13 Partial 0.787 - s Africa, s Indian, Antarctica
The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:
+ New Moon > New Crescent > First Quarter > Waxing Gibbous > Full Moon >
Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)
The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon's 29.5 day period?
To many early civilizations, the Moon's monthly cycle was an important tool for measuring the passage of time. In fact many calendars are synchronized to the phases of the Moon. The Hebrew, Muslem and Chinese calendars are all lunar calendars. The New Moon phase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moon's monthly cycle. When the Moon is New, it rises and sets with the Sun because it lies very close to the Sun in the sky. Although we cannot see the Moon during New Moon phase, it has a very special significance with regard to eclipses.
The Moon's Two Shadows
An eclipse of the Sun (or solar eclipse) can only occur at New Moon when the Moon passes between Earth and Sun. If the Moon's shadow happens to fall upon Earth's surface at that time, we see some portion of the Sun's disk covered or 'eclipsed' by the Moon. Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn't happen because the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region.
The Moon's shadow actually has two parts:
1. Penumbra
+ The Moon's faint outer shadow.
+ Partial solar eclipses are visible from within the penumbral shadow.
2. Umbra
+ The Moon's dark inner shadow.
+ Total solar eclipses are visible from within the umbral shadow.
When the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region. Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright. You must use special filters or a home-made pinhole projector to safely watch a partial eclipse of the Sun (see: Observing Solar Eclipses Safely).
What is the difference between a solar eclipse and a lunar eclipse? A lunar eclipse is an eclipse of the Moon rather than the Sun. It happens when the Moon passes through Earth's shadow. This is only possible when the Moon is in the Full Moon phase. For more information, see Lunar Eclipses for Beginners.
Total Solar Eclipses and the Path of Totality
If the Moon's inner or umbral shadow sweeps across Earth's surface, then a total eclipse of the Sun is seen. The track of the Moon's umbral shadow across Earth is called the Path of Totality. It is typically 10,000 miles long but only about 100 miles wide. It covers less than 1% of Earth's entire surface area. In order to see the Sun become completely eclipsed by the Moon, you must be somewhere inside the narrow path of totality.
The path of a total eclipse can cross any part of Earth. Even the North and South Poles get a total eclipse sooner or later. Just one total eclipse occurs each year or two. Since each total eclipse is only visible from a very narrow track, it is rare to see one from any single location. You'd have to wait an average of 375 years to see two total eclipses from one place. Of course, the interval between seeing two eclipses from one particular place can be shorter or longer. For instance, the last total eclipse visible from Princeton, NJ was in 1478 and the next is in 2079. That's an interval of 601 years. However, the following total eclipse from Princeton is in 2144, after a period of only 65 years.
Awesome Totality
The total phase of a solar eclipse is very brief. It rarely lasts more than several minutes. Nevertheless, it is considered to be one of the most awe inspiring spectacles in all of nature. The sky takes on an eerie twilight as the Sun's bright face is replaced by the black disk of the Moon. Surrounding the Moon is a beautiful gossemer halo. This is the Sun's spectacular solar corona, a super heated plasma two million degrees in temperature. The corona can only be seen during the few brief minutes of totality. To witness such an event is a singularly memorable experience which cannot be conveyed adequately through words or photographs. Nevertheless, you can read more about the Experience of Totality in the first chapter of Totality - Eclipses of the Sun.
Annular Solar Eclipses
Unfortunately, not every eclipse of the Sun is a total eclipse. Sometimes, the Moon is too small to cover the entire Sun's disk. To understand why, we need to talk about the Moon's orbit around Earth. That orbit is not perfectly round but is oval or elliptical in shape. As the Moon orbits our planet, it's distance varies from about 221,000 to 252,000 miles. This 13% variation in the Moon's distance makes the Moon's apparent size in our sky vary by the same amount. When the Moon is on the near side of its orbit, the Moon appears larger than the Sun. If an eclipse occurs at that time, it will be a total eclipse. However, if an eclipse occurs while the Moon is on the far side of its orbit, the Moon appears smaller than the Sun and can't completely cover it. Looking down from space, we would see that the Moon's umbral shadow is not long enough to reach Earth. Instead, the antumbra shadow reaches Earth.
The track of the antumbra is called the path of annularity. If you are within this path, you will see an eclipse where a ring or annulus of bright sunlight surrounds the Moon at the maximum phase. Annular eclipses are also dangerous to look directly with the naked eye. You must use the same precautions needed for safely viewing a partial eclipse of the Sun (see: Observing Solar Eclipses Safely).
Annularity can last as long as a dozen minutes, but is more typically about half that length. Since the annular phase is so bright, the Sun's gorgeous corona remains hidden from view. But annular eclipses are still quite interesting to watch. You can read reports about the annular eclipses of 1999 in Australia and 2003 in Iceland. More recently, visit the 2005 Annular Solar Eclipse Photo Gallery.
The "Oddball" Hybrid Eclipse
There's one more type of solar eclipse to mention and its a real oddball. Under rare circumstances, a total eclipse can change to an annular eclipse or vice versa along different sections of the eclipse path. This happens when the curvature of Earth brings different points of the path into the umbral (total) and antumbral (annular) shadows, respectively. Hybrid eclipses are sometimes called annular/total eclipses. The last hybrid eclipse was in 2005 and the next one is in 2013.
Solar Eclipse Frequency and Future Eclipses
During the five thousand year period 2000 BCE to 3000 CE, planet Earth experiences 11,898 solar eclipses as follows:
Scientists welcome the total eclipse as a rare opportunity to study the Sun's faint corona. Why is the corona so hot? What causes it to spew massive bubbles of plasma into space through coronal mass ejections? Can solar flares be predicted and what causes them? These major mysteries may eventually be solved through experiments performed at future total eclipses.
For amateur astronomers and eclipse chasers, an eclipse of the Sun presents a tempting target to photograph. Fortunately, Solar Eclipse Photography is easy provided that you have the right equipment and use it correctly. See MrEclipse's Picks for camera, lens and tripod recommendations. For more photographs taken during previous lunar eclipses, be sure to visit Solar Eclipse Photo Gallery. It's also possible to capture a solar eclipse using a video camcorder.
The most recent total solar eclipse occurred on March 29, 2006 and was visible from Africa and central Asia. Fred Espenak led a Spears Travel tour to Libya to witness the event. You can see a collection of his photographs at 2006 Eclipse Gallary. Reports (with photos) from some of his earlier eclipse expeditions include 2001 Eclipse in Zambia, 1999 Eclipse in Turkey, 1998 Eclipse in Aruba and 1995 Eclipse in India.
The next two total eclipse of the Sun are both visible from China: 2008 and 2009. Join Fred Espenak on a Spears Travel tour to witness one (or both!) of these spectacular events.
Eclipse conditions
Eclipses may occur when the Earth and the Moon are aligned with the Sun, and the shadow of one body cast by the Sun falls on the other. So at new moon (or rather Dark Moon), when the Moon is in conjunction with the Sun, the Moon may pass in front of the Sun as seen from a narrow region on the surface of the Earth and cause a solar eclipse. At full moon, when the Moon is in opposition to the Sun, the Moon may pass through the shadow of the Earth, and a lunar eclipse is visible from the night half of the Earth.
Note: Conjunction and opposition of the Moon together have a special name: syzygy (from Greek for "junction"), because of the importance of these lunar phases.
An eclipse does not happen at every new or full moon, because the plane of the orbit of the Moon around the Earth is tilted with respect to the plane of the orbit of the Earth around the Sun (the ecliptic): so as seen from the Earth, when the Moon is nearest to the Sun (new moon) or at largest distance (full moon), the three bodies usually are not exactly on the same line.
This inclination is on average about:
A lunar eclipse occurs when the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse. The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes. The next total lunar eclipse will occur on December 21, 2010. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place. Some lunar eclipses have been associated with important historical events.
Types of lunar eclipses
A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.
A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth’s umbra, one observes a total lunar eclipse. The Moon’s speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the Moon’s first and last contact with the shadow is much longer, and could last up to 3.8 hours.[1] The relative distance of the Moon from the Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease much with distance. Thus, a totally eclipsed Moon occurring near apogee will lengthen the duration of totality.
A selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time. This can only happen just before sunset or just after sunrise, and both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being referred to as a horizontal eclipse. It happens during every lunar eclipse at all those places on the Earth where it is sunrise or sunset at the time. Indeed, the reddened light that reaches the Moon comes from all the simultaneous sunrises and sunsets on the Earth. Although the Moon is in the Earth’s geometrical shadow, the Sun and the eclipsed Moon can appear in the sky at the same time because the refraction of light through the Earth’s atmosphere causes objects near the horizon to appear higher in the sky than their true geometric position.
The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth’s atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color; an alternative way of considering the problem is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth.
The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.
Eclipse cycles
Every year there are at least two lunar eclipses, although total lunar eclipses are significantly less common. If one knows the date and time of an eclipse, it is possible to predict the occurrence of other eclipses using an eclipse cycle like the Saros cycle.
Eclipses of the Sun: 2009 - 2015
Calendar Date Eclipse Type Eclipse Magnitude Central Duration Geographic Region of Eclipse Visibility
(Link to Global Map) (Link to Google Map) (Link to Path Table)
2009 Jan 26 Annular 0.928 07m54s s Africa, Antarctica, se Asia, Australia
[Annular: s Indian, Sumatra, Borneo]
2009 Jul 22 Total 1.080 06m39s e Asia, Pacific Ocean, Hawaii
[Total: India, Nepal, China, c Pacific]
2010 Jan 15 Annular 0.919 11m08s Africa, Asia
[Annular: c Africa, India, Malymar, China]
2010 Jul 11 Total 1.058 05m20s s S. America
[Total: s Pacific, Easter Is., Chile, Argentina]
2011 Jan 04 Partial 0.858 - Europe, Africa, c Asia
2011 Jun 01 Partial 0.601 - e Asia, n N. America, Iceland
2011 Jul 01 Partial 0.097 - s Indian Ocean
2011 Nov 25 Partial 0.905 - s Africa, Antarctica, Tasmania, N.Z.
2012 May 20 Annular 0.944 05m46s Asia, Pacific, N. America
[Annular: China, Japan, Pacific, w U.S.]
2012 Nov 13 Total 1.050 04m02s Australia, N.Z., s Pacific, s S. America
[Total: n Australia, s Pacific]
2013 May 10 Annular 0.954 06m03s Australia, N.Z., c Pacific
[Annular: n Australia, Solomon Is., c Pacific]
2013 Nov 03 Hybrid 1.016 01m40s e Americas, s Europe, Africa
[Hybid: Atlantic, c Africa]
2014 Apr 29 Annular 0.987 - s Indian, Australia, Antarctica
[Annular: Antarctica]
2014 Oct 23 Partial 0.811 - n Pacific, N. America
2015 Mar 20 Total 1.045 02m47s Iceland, Europe, n Africa, n Asia
[Total: n Atlantic, Faeroe Is, Svalbard]
2015 Sep 13 Partial 0.787 - s Africa, s Indian, Antarctica
Friday, June 25, 2010
history of astronomy
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
Imagine the Universe! Dictionary
Please allow the whole page to load before you start searching for an entry. Otherwise, errors will occur.
[A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ]
(Note - Greek letters are written out by name - alpha, beta etc.)
A
absorption
The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule.
absorption line spectrum
A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and moons.
accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.
active galactic nuclei (AGN)
A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.
Tell me about AGN!
Tell me more about AGN!
angstrom
A unit of length equal to 0.00000001 centimeters. This may also be written as 1 x 10-8 cm (see scientific notation).
angular momentum
A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.
apastron
The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron.
aphelion
The point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.
apoapsis
The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion, and apogee.
apogee
The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.
arc minute
An angular measurement equal to 1/60th of a degree.
arc second
An angular measurement equal to 1/60th of an arc minute or 1/3600th of a degree.
Ariel V
A UK X-ray mission, also known as UK-5
ASCA
The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
ASD
Astrophysics Science Division, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.
ASM
All Sky Monitor. An instrument designed to observe large areas of the sky for interesting astronomical phenomena. An ASM measures the intensity of many sources across the sky and looks for new sources. Many high-energy satellites have carried ASM detectors, including the ASM on Vela 5B, Ariel V, and the Rossi X-ray Timing Explorer.
Astro-E/Astro-E2
A X-ray/gamma-ray mission built jointly by the United States and Japan. Astro E was destroyed in February 2000, when a Japanese M-5 rocket failed to lift the instrument into orbit. A replacement mission, Astro-E2, was succesfully launched in July 2005, and subsequently renamed Suzaku.
Tell me more about Astro-E.
Tell me more about Astro-E2/Suzaku.
astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.
astronomy
The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
astrophysics
The part of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.
atmosphere
The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.
AXAF
The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra X-ray Observatory, CXO, and launched in July 1999.
Tell me more about AXAF.
B
Balmer lines (J. Balmer)
Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) state and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.
baryon
Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.
BATSE
BATSE (Burst and Transient Source Experiment) was an instrument aboard the Compton Gamma Ray Observatory that detected and located gamma-ray bursts in the sky.
BBXRT
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space shuttle flight (Dec. 1990)
Tell me more about BBXRT.
Big Bang
A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion (referred to as the "Big Bang").
binary stars
Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.
Tell me about X-ray binary stars.
Tell me more about X-ray binary stars.
black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
black hole
An object whose gravity is so strong that not even light can escape from it.
Tell me about X-rays from black holes.
Tell me about gamma rays from black holes and neutron stars.
Tell me more about black holes.
black-hole dynamic laws; laws of black-hole dynamics
1. First law of black hole dynamics:
For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum, hold. This is analogous to the first law of thermodynamics.
2. Second law of black hole dynamics:
With black-hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease. This is analogous to the second law of thermodynamics, with the surface areas of the black holes being a measure of the entropy of the system.
blackbody radiation
Blackbody radiation is produced by an object which is a perfect absorber of heat. Perfect absorbers must also be perfect radiators. For a blackbody at a temperature T, the intensity of radiation emitted I at a particular energy E is given by Plank's law:
I(E,T) = 2 E3[h2c2(eE/kT - 1)]-1
where h is Planck's constant, k is Boltzmann's constant, and c is the the speed of light.
blackbody temperature
The temperature of an object if it is re-radiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not re-radiate all the excess heat and the leftover will go toward increasing its temperature.
blueshift
An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler effect.
bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).
Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 J/K (see scientific notation).
Brahe, Tycho (1546 - 1601)
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Johannes Kepler's laws of planetary motion. Brahe lost his nose in a dual in 1566 with Manderup Parsberg (a fellow student and nobleman) at Rostock over who was the better mathematician. He died in 1601, not of a burst bladder as legend suggests, but from high levels of mercury in his blood (which he may have taken as medication after falling ill from the infamous meal). Show me a picture of Tycho Brahe !
bremsstrahlung
"Braking radiation", the main way very fast charged particles lose energy when traveling through matter. Radiation is emitted when charged particles are accelerated. In this case, the acceleration is caused by the electromagnetic fields of the atomic nuclei of the medium.
C
calibration
A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.
cataclysmic variable (CV)
Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.
Tell me more about Cataclysmic Variables.
Cepheid Variable
A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.
Tell me more about Cepheid Variables.
CGRO
The Compton Gamma Ray Observatory
Tell me more about CGRO.
Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July 1999, and named after S. Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF).
Chandrasekhar, S. (1910 - 1995)
Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other compact objects.
Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.
cluster of galaxies
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.
collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.
Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.
Tell me more about Dr. Compton and the Compton Effect.
Tell me how gamma-ray astronomers use the Compton effect.
Copernicus
NASA ultraviolet/X-ray mission, also known as OAO-3
Tell me more about the Copernicus mission.
Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun (the "heliocentric" theory). This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the center of the universe, and all objects, including the sun, circle it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.). Show me a picture of Nicholas Copernicus !
corona (plural: coronae)
The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K).
Tell me about X-rays from the Sun's corona.
Tell me about X-rays from other stellar coronae.
COS-B
A satellite launched in August 1975 to study extraterrestrial sources of gamma-ray emission.
Tell me more about COS-B.
cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz (see scientific notation) discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself.
cosmic rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies.
cosmological constant; Lambda
A constant term (labeled Lambda) which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting. The cosmological constant was found to be unnecessary once observations indicated the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life".
cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and redshift.
cosmological redshift
An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. Compare Doppler effect.
cosmology
The astrophysical study of the history, structure, and dynamics of the universe.
CXO
The Chandra X-ray Observatory. CXO was launched by the Space Shuttle in July 1999, and named for S. Chandrasekhar.
Tell me more about CXO.
D
dark matter
Name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be. Not one, at the moment is convincing enough and the question is still a mystery.
de Broglie wavelength (L. de Broglie; 1924)
The quantum mechanical "wavelength" associated with a particle, named after the scientist who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.
Declination
A coordinate which, along with Right Ascension, may be used to locate any position in the sky. Declination is analogous to latitude for locating positions on the Earth, and ranges from +90 degrees to -90 degrees.
deconvolution
An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky. For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.
density
The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3; lead is 11.3 gm/cm3.
Dewar
A container (akin to a thermos bottle) that keeps cold material cold. In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (4.2K). Some astronomical detectors work better at cold temperatures.
disk
(a) A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. (b) The apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.
Doppler effect (C.J. Doppler)
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver. Compare cosmological redshift.
dust
Not the dust one finds around the house (which is typically fine bits of fabric, dirt, and dead skin cells). Rather, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across which are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.
dust tail
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.
http://imagine.gsfc.nasa.gov/docs/dict_ad.html#A
Q
quasar
An enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.
quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic and highly luminous.
R
radial velocity
The speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky.
radian; rad
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57o.
radiation
Energy emitted in the form of waves (light) or particles (photons).
radiation belt
Regions of charged particles in a magnetosphere.
radio
Electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth; the atmosphere of the Earth is transparent to radio waves with wavelengths from a few millimeters to about twenty meters.
Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish the location of objects which are near each other. It begins with the assumption that the central ring of one image should fall on the first dark ring of another image; for an objective lens with diameter d and employing light with a wavelength lambda (usually taken to be 560 nm), the resolving power is approximately given by
1.22 x lambda/d
Rayleigh-Taylor instabilities
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is pushed against a light fluid -- like trying to balance water on top of air by filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor instabilities are important in many astronomical objects, because the two fluids trade places by sticking "fingers" into each other. These "fingers" can drag the magnetic field lines along with them, thus both enhancing and aligning the magnetic field. This result is evident in the example of a supernova remnant in the diagram below, from Chevalier (1977):
red giant
A star that has low surface temperature and a diameter that is large relative to the Sun.
redshift
An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. See also Doppler effect.
reflection law
For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
relativity principle
The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity.
relativity, theory of
Theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)
resolution (spatial)
In astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope.
resolution (spectral or frequency)
Similar to spatial resolution except that it applies to frequency, spectral resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.
resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
retrograde
The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies.
revolution
The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."
Right Ascension
A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on the Earth.
Ritter, Johann Wilhelm (1776 - 1810)
Ritter is credited with discovering and investigating the ultraviolet region of the electromagnetic spectrum.
Roche limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break up the smaller object.
Roche lobe
The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star. A particle released above the Roche lobe of either star will, in general, occupy the `circumbinary' region that surrounds both stars. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point at which it `fills' its Roche lobe, theoretical calculations predict that material from this star will overflow both onto the companion star (via the L1 point) and into the environment around the binary system.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
Tell me more about Wilhelm Röntgen
ROSAT
Röntgen Satellite
Tell me more about ROSAT
rotation
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
S
SAS-2
The second Small Astronomy Satellite: a NASA satellite launched November 1972 with a mission dedicated to gamma-ray astronomy.
Tell me more about SAS-2
SAS-3
The third Small Astronomy Satellite: a NASA satellite launched May 1975 to determine the location of bright X-ray sources and search for X-ray novae and other transient phenomena.
Tell me more about SAS-3
satellite
A body that revolves around a larger body. For example, the moon is a satellite of the earth.
Schwarzschild black hole
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.
Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.
scientific notation
A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, and a power of ten. Thus 1.23 x 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.
second; s
The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10-9) of a second.
semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by
rp = a(1-e) and ra = a(1+e).
sensitivity
A measure of how bright objects need to be in order for that telescope to detect these objects. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.
shock wave
A strong compression wave where there is a sudden change in gas velocity, density, pressure and temperature.
singularity
In astronomy, a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity. Mathematically, a singularity is a condition when equations do not give a valid value, and can sometimes be avoided by using a different coordinate system.
soft x-ray
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.
solar flares
Violent eruptions of gas on the Sun's surface.
solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1033 grams.
special relativity
The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also general relativity.
spectral line
Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).
spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
A Dispersive Spectrometer is like a prism. It scatters light of different energies to different places. We measure the energy by noting where the X-rays go. A Non-Dispersive Spectrometer measures the energy directly.
spectroscopy
The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.
Tell me more about spectra
speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,282 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
star
A large ball of gas that creates and emits its own radiation.
star cluster
A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6697 x 10-8 Watts per square meter per degree Kelvin to the fourth power (see scientific notation).
Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power P (rate of emission of electromagnetic energy) of a hot body is proportional to the radiating surface area, A, and the fourth power of the thermodynamic temperature, T. The constant of proportionality is the Stefan-Boltzmann constant.
stellar classification
Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.
stellar wind
The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.
steradian; sr
The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.
supernova (plural: supernovae)
(a)The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).
(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.
Tell me about X-rays from supernovae and their remnants
Tell me about gamma rays from supernovae
Tell me more about supernovae
Tell me more about supernova remnants
sunspots
Cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface.
Suzaku
A Japanese X-ray satellite observatory for which NASA provided X-ray mirrors and an X-ray Spectrometer using a calorimeter design. Suzaku (formerly known as Astro-E2) was successfully launched in July 2005.
Tell me more about Suzaku
SXG
The Spectrum X-Gamma mission
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Swift
Swift is a NASA mid-sized mission whose primary goal is to study gamma-ray bursts and address the mysteries surrounding their nature, origin, and causes. Swift launched November 20, 2004.
Tell me more about Swift
synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.
synchrotron radiation
Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.
Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from the MKS system (which itself is derived from the metric system), in common use in physics today. The fundamental SI unit of length is the meter, of time is the second, and of mass is the kilogram.
T
Tenma
The second Japanese X-ray mission, also known as Astro-B.
Tell me more about Tenma
Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale and who supervised the laying of a trans-Atlantic cable. Show me a picture of Lord Kelvin!
time dilation
The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light, and in regions of high gravity.
U
Uhuru
NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's independence. The satellite was named "Uhuru" (Swahili for "freedom") in honor of its launch date.
Tell me more about Uhuru
ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.
universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity. It is equal to 6.67428 x 10-11 m3 / kg-sec2, a value recommended in 2006 by the Committee on Data for Science and Technology. (Also see scientific notation.)
Universe
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
V
Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an all-sky X-ray monitor
Tell me more about Vela 5B
The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent by the Soviet Union to the planet Venus. Several Venera satellites carried high-energy astrophysics detectors.
Tell me more about Venera 11 & 12
Tell me more about Venera 13 & 14
visible
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)
W
wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.
wavelength
The distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.
white dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!
Tell me more about white dwarfs
Wien's displacement law
For a blackbody, the product of the wavelength corresponding to the maximum radiancy and the thermodynamic temperature is a constant. As a result, as the temperature rises, the maximum of the radiant energy shifts toward the shorter wavelength (higher frequency and energy) end of the spectrum.
WMAP (Wilkinson Microwave Anisotropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.
Tell me more about WMAP
WWW
The World Wide Web -- a loose linkage of Internet sites which provide data and other services from around the world.
X
X-ray
Electromagnetic radiation of very short wavelength and very high-energy; X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.
XSELECT
A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of astronomical data.
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
Tell me more about RXTE
Y
Z
Z
The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by
z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of light, then
z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
Imagine the Universe! Dictionary
Please allow the whole page to load before you start searching for an entry. Otherwise, errors will occur.
[A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ]
(Note - Greek letters are written out by name - alpha, beta etc.)
A
absorption
The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule.
absorption line spectrum
A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and moons.
accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.
active galactic nuclei (AGN)
A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.
Tell me about AGN!
Tell me more about AGN!
angstrom
A unit of length equal to 0.00000001 centimeters. This may also be written as 1 x 10-8 cm (see scientific notation).
angular momentum
A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.
apastron
The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron.
aphelion
The point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.
apoapsis
The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion, and apogee.
apogee
The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.
arc minute
An angular measurement equal to 1/60th of a degree.
arc second
An angular measurement equal to 1/60th of an arc minute or 1/3600th of a degree.
Ariel V
A UK X-ray mission, also known as UK-5
ASCA
The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
ASD
Astrophysics Science Division, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.
ASM
All Sky Monitor. An instrument designed to observe large areas of the sky for interesting astronomical phenomena. An ASM measures the intensity of many sources across the sky and looks for new sources. Many high-energy satellites have carried ASM detectors, including the ASM on Vela 5B, Ariel V, and the Rossi X-ray Timing Explorer.
Astro-E/Astro-E2
A X-ray/gamma-ray mission built jointly by the United States and Japan. Astro E was destroyed in February 2000, when a Japanese M-5 rocket failed to lift the instrument into orbit. A replacement mission, Astro-E2, was succesfully launched in July 2005, and subsequently renamed Suzaku.
Tell me more about Astro-E.
Tell me more about Astro-E2/Suzaku.
astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.
astronomy
The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
astrophysics
The part of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.
atmosphere
The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.
AXAF
The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra X-ray Observatory, CXO, and launched in July 1999.
Tell me more about AXAF.
B
Balmer lines (J. Balmer)
Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) state and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.
baryon
Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.
BATSE
BATSE (Burst and Transient Source Experiment) was an instrument aboard the Compton Gamma Ray Observatory that detected and located gamma-ray bursts in the sky.
BBXRT
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space shuttle flight (Dec. 1990)
Tell me more about BBXRT.
Big Bang
A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion (referred to as the "Big Bang").
binary stars
Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.
Tell me about X-ray binary stars.
Tell me more about X-ray binary stars.
black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
black hole
An object whose gravity is so strong that not even light can escape from it.
Tell me about X-rays from black holes.
Tell me about gamma rays from black holes and neutron stars.
Tell me more about black holes.
black-hole dynamic laws; laws of black-hole dynamics
1. First law of black hole dynamics:
For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum, hold. This is analogous to the first law of thermodynamics.
2. Second law of black hole dynamics:
With black-hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease. This is analogous to the second law of thermodynamics, with the surface areas of the black holes being a measure of the entropy of the system.
blackbody radiation
Blackbody radiation is produced by an object which is a perfect absorber of heat. Perfect absorbers must also be perfect radiators. For a blackbody at a temperature T, the intensity of radiation emitted I at a particular energy E is given by Plank's law:
I(E,T) = 2 E3[h2c2(eE/kT - 1)]-1
where h is Planck's constant, k is Boltzmann's constant, and c is the the speed of light.
blackbody temperature
The temperature of an object if it is re-radiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not re-radiate all the excess heat and the leftover will go toward increasing its temperature.
blueshift
An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler effect.
bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).
Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 J/K (see scientific notation).
Brahe, Tycho (1546 - 1601)
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Johannes Kepler's laws of planetary motion. Brahe lost his nose in a dual in 1566 with Manderup Parsberg (a fellow student and nobleman) at Rostock over who was the better mathematician. He died in 1601, not of a burst bladder as legend suggests, but from high levels of mercury in his blood (which he may have taken as medication after falling ill from the infamous meal). Show me a picture of Tycho Brahe !
bremsstrahlung
"Braking radiation", the main way very fast charged particles lose energy when traveling through matter. Radiation is emitted when charged particles are accelerated. In this case, the acceleration is caused by the electromagnetic fields of the atomic nuclei of the medium.
C
calibration
A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.
cataclysmic variable (CV)
Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.
Tell me more about Cataclysmic Variables.
Cepheid Variable
A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.
Tell me more about Cepheid Variables.
CGRO
The Compton Gamma Ray Observatory
Tell me more about CGRO.
Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July 1999, and named after S. Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF).
Chandrasekhar, S. (1910 - 1995)
Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other compact objects.
Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.
cluster of galaxies
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.
collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.
Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.
Tell me more about Dr. Compton and the Compton Effect.
Tell me how gamma-ray astronomers use the Compton effect.
Copernicus
NASA ultraviolet/X-ray mission, also known as OAO-3
Tell me more about the Copernicus mission.
Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun (the "heliocentric" theory). This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the center of the universe, and all objects, including the sun, circle it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.). Show me a picture of Nicholas Copernicus !
corona (plural: coronae)
The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K).
Tell me about X-rays from the Sun's corona.
Tell me about X-rays from other stellar coronae.
COS-B
A satellite launched in August 1975 to study extraterrestrial sources of gamma-ray emission.
Tell me more about COS-B.
cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz (see scientific notation) discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself.
cosmic rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies.
cosmological constant; Lambda
A constant term (labeled Lambda) which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting. The cosmological constant was found to be unnecessary once observations indicated the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life".
cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and redshift.
cosmological redshift
An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. Compare Doppler effect.
cosmology
The astrophysical study of the history, structure, and dynamics of the universe.
CXO
The Chandra X-ray Observatory. CXO was launched by the Space Shuttle in July 1999, and named for S. Chandrasekhar.
Tell me more about CXO.
D
dark matter
Name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be. Not one, at the moment is convincing enough and the question is still a mystery.
de Broglie wavelength (L. de Broglie; 1924)
The quantum mechanical "wavelength" associated with a particle, named after the scientist who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.
Declination
A coordinate which, along with Right Ascension, may be used to locate any position in the sky. Declination is analogous to latitude for locating positions on the Earth, and ranges from +90 degrees to -90 degrees.
deconvolution
An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky. For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.
density
The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3; lead is 11.3 gm/cm3.
Dewar
A container (akin to a thermos bottle) that keeps cold material cold. In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (4.2K). Some astronomical detectors work better at cold temperatures.
disk
(a) A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. (b) The apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.
Doppler effect (C.J. Doppler)
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver. Compare cosmological redshift.
dust
Not the dust one finds around the house (which is typically fine bits of fabric, dirt, and dead skin cells). Rather, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across which are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.
dust tail
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.
http://imagine.gsfc.nasa.gov/docs/dict_ad.html#A
Q
quasar
An enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.
quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic and highly luminous.
R
radial velocity
The speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky.
radian; rad
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57o.
radiation
Energy emitted in the form of waves (light) or particles (photons).
radiation belt
Regions of charged particles in a magnetosphere.
radio
Electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth; the atmosphere of the Earth is transparent to radio waves with wavelengths from a few millimeters to about twenty meters.
Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish the location of objects which are near each other. It begins with the assumption that the central ring of one image should fall on the first dark ring of another image; for an objective lens with diameter d and employing light with a wavelength lambda (usually taken to be 560 nm), the resolving power is approximately given by
1.22 x lambda/d
Rayleigh-Taylor instabilities
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is pushed against a light fluid -- like trying to balance water on top of air by filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor instabilities are important in many astronomical objects, because the two fluids trade places by sticking "fingers" into each other. These "fingers" can drag the magnetic field lines along with them, thus both enhancing and aligning the magnetic field. This result is evident in the example of a supernova remnant in the diagram below, from Chevalier (1977):
red giant
A star that has low surface temperature and a diameter that is large relative to the Sun.
redshift
An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. See also Doppler effect.
reflection law
For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
relativity principle
The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity.
relativity, theory of
Theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)
resolution (spatial)
In astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope.
resolution (spectral or frequency)
Similar to spatial resolution except that it applies to frequency, spectral resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.
resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
retrograde
The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies.
revolution
The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."
Right Ascension
A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on the Earth.
Ritter, Johann Wilhelm (1776 - 1810)
Ritter is credited with discovering and investigating the ultraviolet region of the electromagnetic spectrum.
Roche limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break up the smaller object.
Roche lobe
The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star. A particle released above the Roche lobe of either star will, in general, occupy the `circumbinary' region that surrounds both stars. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point at which it `fills' its Roche lobe, theoretical calculations predict that material from this star will overflow both onto the companion star (via the L1 point) and into the environment around the binary system.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
Tell me more about Wilhelm Röntgen
ROSAT
Röntgen Satellite
Tell me more about ROSAT
rotation
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
S
SAS-2
The second Small Astronomy Satellite: a NASA satellite launched November 1972 with a mission dedicated to gamma-ray astronomy.
Tell me more about SAS-2
SAS-3
The third Small Astronomy Satellite: a NASA satellite launched May 1975 to determine the location of bright X-ray sources and search for X-ray novae and other transient phenomena.
Tell me more about SAS-3
satellite
A body that revolves around a larger body. For example, the moon is a satellite of the earth.
Schwarzschild black hole
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.
Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.
scientific notation
A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, and a power of ten. Thus 1.23 x 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.
second; s
The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10-9) of a second.
semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by
rp = a(1-e) and ra = a(1+e).
sensitivity
A measure of how bright objects need to be in order for that telescope to detect these objects. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.
shock wave
A strong compression wave where there is a sudden change in gas velocity, density, pressure and temperature.
singularity
In astronomy, a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity. Mathematically, a singularity is a condition when equations do not give a valid value, and can sometimes be avoided by using a different coordinate system.
soft x-ray
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.
solar flares
Violent eruptions of gas on the Sun's surface.
solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1033 grams.
special relativity
The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also general relativity.
spectral line
Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).
spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
A Dispersive Spectrometer is like a prism. It scatters light of different energies to different places. We measure the energy by noting where the X-rays go. A Non-Dispersive Spectrometer measures the energy directly.
spectroscopy
The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.
Tell me more about spectra
speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,282 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
star
A large ball of gas that creates and emits its own radiation.
star cluster
A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6697 x 10-8 Watts per square meter per degree Kelvin to the fourth power (see scientific notation).
Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power P (rate of emission of electromagnetic energy) of a hot body is proportional to the radiating surface area, A, and the fourth power of the thermodynamic temperature, T. The constant of proportionality is the Stefan-Boltzmann constant.
stellar classification
Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.
stellar wind
The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.
steradian; sr
The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.
supernova (plural: supernovae)
(a)The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).
(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.
Tell me about X-rays from supernovae and their remnants
Tell me about gamma rays from supernovae
Tell me more about supernovae
Tell me more about supernova remnants
sunspots
Cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface.
Suzaku
A Japanese X-ray satellite observatory for which NASA provided X-ray mirrors and an X-ray Spectrometer using a calorimeter design. Suzaku (formerly known as Astro-E2) was successfully launched in July 2005.
Tell me more about Suzaku
SXG
The Spectrum X-Gamma mission
Tell me more about SXG
Swift
Swift is a NASA mid-sized mission whose primary goal is to study gamma-ray bursts and address the mysteries surrounding their nature, origin, and causes. Swift launched November 20, 2004.
Tell me more about Swift
synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.
synchrotron radiation
Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.
Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from the MKS system (which itself is derived from the metric system), in common use in physics today. The fundamental SI unit of length is the meter, of time is the second, and of mass is the kilogram.
T
Tenma
The second Japanese X-ray mission, also known as Astro-B.
Tell me more about Tenma
Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale and who supervised the laying of a trans-Atlantic cable. Show me a picture of Lord Kelvin!
time dilation
The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light, and in regions of high gravity.
U
Uhuru
NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's independence. The satellite was named "Uhuru" (Swahili for "freedom") in honor of its launch date.
Tell me more about Uhuru
ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.
universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity. It is equal to 6.67428 x 10-11 m3 / kg-sec2, a value recommended in 2006 by the Committee on Data for Science and Technology. (Also see scientific notation.)
Universe
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
V
Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an all-sky X-ray monitor
Tell me more about Vela 5B
The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent by the Soviet Union to the planet Venus. Several Venera satellites carried high-energy astrophysics detectors.
Tell me more about Venera 11 & 12
Tell me more about Venera 13 & 14
visible
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)
W
wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.
wavelength
The distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.
white dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!
Tell me more about white dwarfs
Wien's displacement law
For a blackbody, the product of the wavelength corresponding to the maximum radiancy and the thermodynamic temperature is a constant. As a result, as the temperature rises, the maximum of the radiant energy shifts toward the shorter wavelength (higher frequency and energy) end of the spectrum.
WMAP (Wilkinson Microwave Anisotropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.
Tell me more about WMAP
WWW
The World Wide Web -- a loose linkage of Internet sites which provide data and other services from around the world.
X
X-ray
Electromagnetic radiation of very short wavelength and very high-energy; X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.
XSELECT
A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of astronomical data.
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
Tell me more about RXTE
Y
Z
Z
The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by
z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of light, then
z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.
English for Specific Purposes: What does it mean? Why is it different? Laurence Anthony
Dept. of Information and Computer Engineering, Faculty of Engineering
Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan
anthony 'at' ice.ous.ac.jp
1. Growth of ESP
From the early 1960's, English for Specific Purposes (ESP) has grown to become one of the most prominent areas of EFL teaching today. Its development is reflected in the increasing number of universities offering an MA in ESP (e.g. The University of Birmingham, and Aston University in the UK) and in the number of ESP courses offered to overseas students in English speaking countries. There is now a well-established international journal dedicated to ESP discussion, "English for Specific Purposes: An international journal", and the ESP SIG groups of the IATEFL and TESOL are always active at their national conferences.
In Japan too, the ESP movement has shown a slow but definite growth over the past few years. In particular, increased interest has been spurred as a result of the Mombusho's decision in 1994 to largely hand over control of university curriculums to the universities themselves. This has led to a rapid growth in English courses aimed at specific disciplines, e.g. English for Chemists, in place of the more traditional 'General English' courses. The ESP community in Japan has also become more defined, with the JACET ESP SIG set up in 1996 (currently with 28 members) and the JALT N-SIG to be formed shortly. Finally, on November 8th this year the ESP community came together as a whole at the first Japan Conference on English for Specific Purposes, held on the campus of Aizu University, Fukushima Prefecture.
2. What is ESP?
As described above, ESP has had a relatively long time to mature and so we would expect the ESP community to have a clear idea about what ESP means. Strangely, however, this does not seem to be the case. In October this year, for example, a very heated debate took place on the TESP-L e-mail discussion list about whether or not English for Academic Purposes (EAP) could be considered part of ESP in general. At the Japan Conference on ESP also, clear differences in how people interpreted the meaning of ESP could be seen. Some people described ESP as simply being the teaching of English for any purpose that could be specified. Others, however, were more precise, describing it as the teaching of English used in academic studies or the teaching of English for vocational or professional purposes.
At the conference, guests were honored to have as the main speaker, Tony Dudley-Evans, co-editor of the ESP Journal mentioned above. Very aware of the current confusion amongst the ESP community in Japan, Dudley-Evans set out in his one hour speech to clarify the meaning of ESP, giving an extended definition of ESP in terms of 'absolute' and 'variable' characteristics (see below).
Definition of ESP (Dudley-Evans, 1997)
Absolute Characteristics
1. ESP is defined to meet specific needs of the learners
2. ESP makes use of underlying methodology and activities of the discipline it serves
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable Characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
The definition Dudley-Evans offers is clearly influenced by that of Strevens (1988), although he has improved it substantially by removing the absolute characteristic that ESP is "in contrast with 'General English'" (Johns et al., 1991: 298), and has included more variable characteristics. The division of ESP into absolute and variable characteristics, in particular, is very helpful in resolving arguments about what is and is not ESP. From the definition, we can see that ESP can but is not necessarily concerned with a specific discipline, nor does it have to be aimed at a certain age group or ability range. ESP should be seen simple as an 'approach' to teaching, or what Dudley-Evans describes as an 'attitude of mind'. This is a similar conclusion to that made by Hutchinson et al. (1987:19) who state, "ESP is an approach to language teaching in which all decisions as to content and method are based on the learner's reason for learning".
3. Is ESP different to General English?
If we agree with this definition,, we begin to see how broad ESP really is. In fact, one may ask 'What is the difference between the ESP and General English approach?' Hutchinson et al. (1987:53) answer this quite simply, "in theory nothing, in practice a great deal". When their book was written, of course, the last statement was quite true. At the time, teachers of General English courses, while acknowledging that students had a specific purpose for studying English, would rarely conduct a needs analysis to find out what was necessary to actually achieve it. Teachers nowadays, however, are much more aware of the importance of needs analysis, and certainly materials writers think very carefully about the goals of learners at all stages of materials production. Perhaps this demonstrates the influence that the ESP approach has had on English teaching in general. Clearly the line between where General English courses stop and ESP courses start has become very vague indeed.
Rather ironically, while many General English teachers can be described as using an ESP approach, basing their syllabi on a learner needs analysis and their own specialist knowledge of using English for real communication, it is the majority of so-called ESP teachers that are using an approach furthest from that described above. Instead of conducting interviews with specialists in the field, analyzing the language that is required in the profession, or even conducting students' needs analysis, many ESP teachers have become slaves of the published textbooks available, unable to evaluate their suitability based on personal experience, and unwilling to do the necessary analysis of difficult specialist texts to verify their contents.
4. The Future of ESP
If the ESP community hopes to grow and flourish in the future, it is vital that the community as a whole understands what ESP actually represents. Only then, can new members join with confidence, and existing members carry on the practices which have brought ESP to the position it has in EFL teaching today. In Japan in particular, ESP is still in its infancy and so now is the ideal time to form such a consensus. Perhaps this can stem from the Dudley-Evans' definition given in this article but I suspect a more rigorous version will be coming soon, in his book on ESP to be published in 1998. Of course, interested parties are also strongly urged to attend the next Japan Conference on ESP, which is certain to focus again on this topic.
Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan
anthony 'at' ice.ous.ac.jp
1. Growth of ESP
From the early 1960's, English for Specific Purposes (ESP) has grown to become one of the most prominent areas of EFL teaching today. Its development is reflected in the increasing number of universities offering an MA in ESP (e.g. The University of Birmingham, and Aston University in the UK) and in the number of ESP courses offered to overseas students in English speaking countries. There is now a well-established international journal dedicated to ESP discussion, "English for Specific Purposes: An international journal", and the ESP SIG groups of the IATEFL and TESOL are always active at their national conferences.
In Japan too, the ESP movement has shown a slow but definite growth over the past few years. In particular, increased interest has been spurred as a result of the Mombusho's decision in 1994 to largely hand over control of university curriculums to the universities themselves. This has led to a rapid growth in English courses aimed at specific disciplines, e.g. English for Chemists, in place of the more traditional 'General English' courses. The ESP community in Japan has also become more defined, with the JACET ESP SIG set up in 1996 (currently with 28 members) and the JALT N-SIG to be formed shortly. Finally, on November 8th this year the ESP community came together as a whole at the first Japan Conference on English for Specific Purposes, held on the campus of Aizu University, Fukushima Prefecture.
2. What is ESP?
As described above, ESP has had a relatively long time to mature and so we would expect the ESP community to have a clear idea about what ESP means. Strangely, however, this does not seem to be the case. In October this year, for example, a very heated debate took place on the TESP-L e-mail discussion list about whether or not English for Academic Purposes (EAP) could be considered part of ESP in general. At the Japan Conference on ESP also, clear differences in how people interpreted the meaning of ESP could be seen. Some people described ESP as simply being the teaching of English for any purpose that could be specified. Others, however, were more precise, describing it as the teaching of English used in academic studies or the teaching of English for vocational or professional purposes.
At the conference, guests were honored to have as the main speaker, Tony Dudley-Evans, co-editor of the ESP Journal mentioned above. Very aware of the current confusion amongst the ESP community in Japan, Dudley-Evans set out in his one hour speech to clarify the meaning of ESP, giving an extended definition of ESP in terms of 'absolute' and 'variable' characteristics (see below).
Definition of ESP (Dudley-Evans, 1997)
Absolute Characteristics
1. ESP is defined to meet specific needs of the learners
2. ESP makes use of underlying methodology and activities of the discipline it serves
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable Characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
The definition Dudley-Evans offers is clearly influenced by that of Strevens (1988), although he has improved it substantially by removing the absolute characteristic that ESP is "in contrast with 'General English'" (Johns et al., 1991: 298), and has included more variable characteristics. The division of ESP into absolute and variable characteristics, in particular, is very helpful in resolving arguments about what is and is not ESP. From the definition, we can see that ESP can but is not necessarily concerned with a specific discipline, nor does it have to be aimed at a certain age group or ability range. ESP should be seen simple as an 'approach' to teaching, or what Dudley-Evans describes as an 'attitude of mind'. This is a similar conclusion to that made by Hutchinson et al. (1987:19) who state, "ESP is an approach to language teaching in which all decisions as to content and method are based on the learner's reason for learning".
3. Is ESP different to General English?
If we agree with this definition,, we begin to see how broad ESP really is. In fact, one may ask 'What is the difference between the ESP and General English approach?' Hutchinson et al. (1987:53) answer this quite simply, "in theory nothing, in practice a great deal". When their book was written, of course, the last statement was quite true. At the time, teachers of General English courses, while acknowledging that students had a specific purpose for studying English, would rarely conduct a needs analysis to find out what was necessary to actually achieve it. Teachers nowadays, however, are much more aware of the importance of needs analysis, and certainly materials writers think very carefully about the goals of learners at all stages of materials production. Perhaps this demonstrates the influence that the ESP approach has had on English teaching in general. Clearly the line between where General English courses stop and ESP courses start has become very vague indeed.
Rather ironically, while many General English teachers can be described as using an ESP approach, basing their syllabi on a learner needs analysis and their own specialist knowledge of using English for real communication, it is the majority of so-called ESP teachers that are using an approach furthest from that described above. Instead of conducting interviews with specialists in the field, analyzing the language that is required in the profession, or even conducting students' needs analysis, many ESP teachers have become slaves of the published textbooks available, unable to evaluate their suitability based on personal experience, and unwilling to do the necessary analysis of difficult specialist texts to verify their contents.
4. The Future of ESP
If the ESP community hopes to grow and flourish in the future, it is vital that the community as a whole understands what ESP actually represents. Only then, can new members join with confidence, and existing members carry on the practices which have brought ESP to the position it has in EFL teaching today. In Japan in particular, ESP is still in its infancy and so now is the ideal time to form such a consensus. Perhaps this can stem from the Dudley-Evans' definition given in this article but I suspect a more rigorous version will be coming soon, in his book on ESP to be published in 1998. Of course, interested parties are also strongly urged to attend the next Japan Conference on ESP, which is certain to focus again on this topic.
history f esp
English for Specific Purposes (ESP) is a sphere of teaching English language including technical English, scientific English, English for medical professionals, English for waiters, and English for tourism[1]. Aviation English as ESP is taught to pilots, air traffic controllers and civil aviation cadets who are going to use it in radio communications [2]. ESP can be also considered as an avatar of language for specific purposes.[3]
Absolute characteristics
1. ESP is defined to meet specific needs of the learners.
2. ESP makes use of underlying methodology and activities of the discipline it serves.
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
Teaching
ESP is taught in many universities of the world. Many professional associations of teachers of English (TESOL, IATEFL) have ESP sections. Much attention is devoted to ESP course design [4], [5]. ESP teaching has much in common with English as a Foreign or Second Language and English for Academic Purposes (EAP). Quickly developing Business English can be considered as part of a larger concept of English for Specific Purposes.
Absolute characteristics
1. ESP is defined to meet specific needs of the learners.
2. ESP makes use of underlying methodology and activities of the discipline it serves.
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
Teaching
ESP is taught in many universities of the world. Many professional associations of teachers of English (TESOL, IATEFL) have ESP sections. Much attention is devoted to ESP course design [4], [5]. ESP teaching has much in common with English as a Foreign or Second Language and English for Academic Purposes (EAP). Quickly developing Business English can be considered as part of a larger concept of English for Specific Purposes.\
History
The idea of this web-based journal was first voiced around 2000 by Simon Winetroube, then English Language Teaching Projects' Officer at the British Council, Russia. It was later discussed at the 8th ESP Anti-Conference in St. Petersburg, Russia in October, 2001.
This publication became possible due to the support that came from the English Language Teaching Contacts Scheme and the British Council, Russia. The first issue appeared in May 2002.
TEACHING ENGLISH FOR SPECIFIC PURPOSES
Fall 2004
Instructor: Dr. Senem Yildiz
syildiz@indiana.edu
English for Specific Purposes (ESP) is known as a learner-centered approach to teaching English as a foreign or second language. It meets the needs of (mostly) adult learners who need to learn a foreign language for use in their specific fields, such as science, technology, medicine, leisure, and academic learning. This course is recommended for graduate students and foreign and second language professionals who wish to learn how to design ESP courses and programs in an area of specialization such as English for business, for Civil Engineering, for Academic Purposes, and for health service purposes. In addition, they are introduced to ESP instructional strategies, materials adaptation and development, and evaluation.
Absolute characteristics
1. ESP is defined to meet specific needs of the learners.
2. ESP makes use of underlying methodology and activities of the discipline it serves.
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
Teaching
ESP is taught in many universities of the world. Many professional associations of teachers of English (TESOL, IATEFL) have ESP sections. Much attention is devoted to ESP course design [4], [5]. ESP teaching has much in common with English as a Foreign or Second Language and English for Academic Purposes (EAP). Quickly developing Business English can be considered as part of a larger concept of English for Specific Purposes.
Absolute characteristics
1. ESP is defined to meet specific needs of the learners.
2. ESP makes use of underlying methodology and activities of the discipline it serves.
3. ESP is centered on the language appropriate to these activities in terms of grammar, lexis, register, study skills, discourse and genre.
Variable characteristics
1. ESP may be related to or designed for specific disciplines
2. ESP may use, in specific teaching situations, a different methodology from that of General English
3. ESP is likely to be designed for adult learners, either at a tertiary level institution or in a professional work situation. It could, however, be for learners at secondary school level
4. ESP is generally designed for intermediate or advanced students.
5. Most ESP courses assume some basic knowledge of the language systems
Teaching
ESP is taught in many universities of the world. Many professional associations of teachers of English (TESOL, IATEFL) have ESP sections. Much attention is devoted to ESP course design [4], [5]. ESP teaching has much in common with English as a Foreign or Second Language and English for Academic Purposes (EAP). Quickly developing Business English can be considered as part of a larger concept of English for Specific Purposes.\
History
The idea of this web-based journal was first voiced around 2000 by Simon Winetroube, then English Language Teaching Projects' Officer at the British Council, Russia. It was later discussed at the 8th ESP Anti-Conference in St. Petersburg, Russia in October, 2001.
This publication became possible due to the support that came from the English Language Teaching Contacts Scheme and the British Council, Russia. The first issue appeared in May 2002.
TEACHING ENGLISH FOR SPECIFIC PURPOSES
Fall 2004
Instructor: Dr. Senem Yildiz
syildiz@indiana.edu
English for Specific Purposes (ESP) is known as a learner-centered approach to teaching English as a foreign or second language. It meets the needs of (mostly) adult learners who need to learn a foreign language for use in their specific fields, such as science, technology, medicine, leisure, and academic learning. This course is recommended for graduate students and foreign and second language professionals who wish to learn how to design ESP courses and programs in an area of specialization such as English for business, for Civil Engineering, for Academic Purposes, and for health service purposes. In addition, they are introduced to ESP instructional strategies, materials adaptation and development, and evaluation.
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